The present invention is directed to quantum optical methods of and apparatus for writing of Bragg reflection filters in optical waveguides and in micro-optic structures. More particularly, this invention is directed to such methods and apparatus for use in applications where a high degree of writing flexibility is needed and particularly when using pulsed laser systems configured for large volume Bragg reflection filter production for Dense Wavelength Division Multiplexing (DWDM).
The present phase mask based technique for Bragg filter fabrication utilizes a mask made of a monolithic material such as fused silica glass. Phase masks are relatively expensive, presently ranging in cost from $2000 to $6,000 each depending upon custom requirements and the manufacturer. This is because they are pre-etched with parallel grooves of a given depth to produce an interference region from selected diffraction orders in the vacinity of the mask to provide a grating pattern in the form of a Bragg reflection filter in an optical fiber inserted into the same region. Each time it is desired to change the wavelength of the Bragg reflection filter using this technique, it is necessary to provide another phase mask at a cost in the thousands of dollars. Consequently, to provide an optical waveguide with a number of Bragg reflection filters rapidly becomes a very expensive proposition. Accordingly, there is a need for methods and apparatus for reducing the cost of providing optical waveguides with Bragg reflection filters of various characteristics.
It is a feature of the present invention to make laser systems, such as CW and pulsed laser systems, for large volume Bragg filter production tunable over a large number of wavelengths for single phase mask units.
In view of this feature, the present invention is directed to a method for fabrication of Bragg reflection filters by providing a wavelength tunable phase mask, in which the phase mask responds more strongly to a first wavelength of laser light than to a second wavelength of laser light. In practicing the method, first beams of laser light are directed into the phase mask unit at acute angles with respect to a reference line to create an interference region within the phase mask. A second beam of laser light is then directed into the interference region of the phase mask created by the first beams to generate diffraction orders. A receptor of a parasitic material is positioned optically proximate the tunable phase mask for writing Bragg reflection filters into the receptor with the diffraction orders that are produced by the interaction between the second beam and the optically altered or index of refraction modulated state of the tunable phase mask. According to the present invention, the characteristics of the Bragg reflection filters in the receptor are changed by altering the characteristics of the interference region created in the phase mask by altering the intensity, i.e., time-averaged square of the amplitude and Bragg angles of the first beams. In accordance with the process, the first beams serve as writing beams while the second beam serves as a reading beam which reads the phase mask pattern and produces a written interference pattern into the receptor to create a permanent Bragg reflection filter therein.
In accordance with the apparatus for practicing the aforedescribed method, the apparatus comprises a tunable phase mask which does not have a particular interference pattern fixed therein, but rather has a selected interference pattern created therein by the interference of two laser light beams directed into the phase material. The selected interference pattern can be changed by altering the characteristics of the laser light comprising the first beams. For example, by altering the Bragg angles and/or amplitude, i.e., intensity of the laser light, one can change the pitch of the phase mask and diffraction order amplitudes of the reading beam. In a specific example of the apparatus, laser light from a single first source is passed through a beam splitter to create two divergent beams. These divergent beams are then reflected from a pair of mirrors so as to converge at a selected interference region within the phase material to write or create a specific, temporary interference pattern therein. The written interference pattern in the phase mask is then read into the receptor to create a selected, permanent Bragg filter therein by the second laser light source.
In accordance with a first aspect of the invention, the receptor of the Bragg reflection filters is an optical waveguide in the form of an optical fiber. However, in other aspects of the invention, the optical waveguide can have other physical configurations and shapes and be made of various optical waveguide materials.
In accordance with one aspect of the invention, the phase mask is a non-linear solid structure similar to a lithium niobate crystalline material in its response to an intense laser beam, but more akin to highly non-linear glasses such as Pb/Si Silicates, Pb/Bi Gallates, Ti/Bi Gallates, Tellurites, Ti/Bi Germanates, Sulfides, etc. such as set forth in N. F. Borrelli et al., xe2x80x9cResonant and Non-Resonant Effects in Photomic Glassesxe2x80x9d, Journal of Non-Crystalline Solids 185O (1995) 109-122, incorporated herein in its entirety by reference.
In still a further aspect of the invention, a phase mask may be quantum fluid material such as, for example, but not limited to, liquid helium. For any quantum fluid, i.e. a fluid system containing a super fluid and a normal fluid component, optical density fluctuations may be induced by the interaction of laser light with the normal and super fluid components.